Dear Editor

Influenza vaccination is critical to public health, and vaccine uptake should be widely encouraged. The effects of vaccination on non-influenza-specific outcomes—for example, on risk of acute myocardial infarction (AMI)—nonetheless remain controversial. Several studies reported no difference in either initial myocardial infarction1 or recurrent coronary events among individuals who received influenza vaccination relative to those who did not.2 However, a recent meta-analysis of randomised control trials suggests that influenza vaccination may offer additional benefit by preventing cardiac events among high-risk patients.3

The recent observational study of ischaemic heart disease and influenza by MacIntyre et al4 also suggests a substantial benefit of vaccination on AMI in a more general population. The critical result of this study is that influenza vaccination dramatically reduces risk of AMI, reported as an OR of 0.55 in Table 3.4 This table also reports that influenza infection itself is not associated with AMI. The contrast of these results, derived from a single multivariate regression model, raises several methodological concerns.

The obvious mechanism by which influenza vaccine might prevent AMI is by preventing influenza infection; that is, infection is part of the hypothesised …